1. Field of the Invention
The present invention relates to an illuminating device that uses LEDs (light emitting diodes) as a light source and more particularly to an LED illuminating device used for, for instance, visual inspection purposes.
2. Prior Art
Illuminating light sources such as fluorescent lamps and halogen light bulbs, etc. have been used in various conventional illuminating devices for the inspection of lead bending, directional discrimination of polarity marks or discrimination of molding defects or cracking in semiconductor manufacturing devices, for dimensional inspection or inspection for scratches in glass and metal product manufacturing lines, etc. Such illuminating devices are also used by being incorporated into an inspection apparatus for inspecting foodstuffs and drug products such as tablets, etc.
However, such light sources generally have problems. They generate a large amount of heat, they have poor resistance to vibration, and they consume a large amount of power. In addition, illuminating devices which use such light sources require the replacement of burned-out lamps, and they are generally large in size. In addition, there is a large variation in the quantity of light caused by voltage fluctuations. Thus, such light sources are not quite adequate in terms of practical utility.
Accordingly, a change from conventional light sources to illuminating devices that use LED light sources has been tentatively explored as a means to solve the problems above.
However, LEDs generally emit only a small quantity of light. It is, therefore, necessary to use means for compensating for this small light quantity. One way is to increase the number of LEDs to be used, and it is also effective to use condensing type LEDs. Various types of lenses have also been employed to condense and diffuse the illuminating light with various types of lenses, thus increasing the apparent size of the light source. These methods are usually used singularly or in combinations.
FIG. 4 illustrates a construction of a conventional illumination device disclosed in Japanese Utility Model Application Laid-Open (Kokai) No. 6-68205.
In this structure, four spot illuminating means 91 are installed on an attachment plate 92 so that these spot illuminating means 91 are inclined toward the center axis, thus producing superimposed LED spotlight 93. In this case, however, correction of the angles of the spot illuminating means 91 is required, and each illuminating means are also large in size.
Currently, monochromatic LEDs of various colors such as red LEDs, yellow LEDs, green LEDs and blue LEDs have been proposed; and such LEDs are used as an illuminating light source for obtaining a specific color light. In the case of these illuminating light sources used for inspections of, for instance, a bend in the leads of a semiconductor device, there is an optimal color of illumination and light quantity depending upon each object to be inspected. In this regard, the problem with the monochromatic LED illuminating devices is that such devices cannot be used as all-purpose light sources for various types of inspections. This is because in cases where visual recognition is accomplished by inspecting digitalized images from, for instance, a CCD camera, it is essential to obtain a high contrast in order to improve the precision of inspection.
Furthermore, in the device shown in FIG. 4, the distribution of the illumination that is projected onto the illuminated surface from the spot illuminating means 91 tends to vary. The reason for this is that the illumination distribution of the LEDs themselves is affected by the internal LED chips and by the shape of the resin molding, etc. that surrounds the LED chips; in addition, if there are differences among respective colors, or even in LEDs of the same type, such differences can cause irregularities in the illumination distribution such as irregularities in the illuminated area and intensity of illumination. Furthermore, there may also be irregularities in the intensity of illumination obtained for reasons related to the structure of the LEDs, e. g., the forming of an image of the internal LED chip on the illuminated surface, etc. For such reasons, even if spotlight are obtained by collecting LED light "as is", there is some unevenness in the illumination distribution of the spotlights depending on the individual LEDs.
Consequently, in the prior art described above, because of irregular illumination in the superimposed LED spotlights 93, and also because of the presence of excess illuminating light in the areas where the spotlights are not superimposed, a high contrast is not obtained, and there is a lack of sharpness in the visual recognition characteristics.